The first body fossils of trilobites appeared in the Early
Cambrian
Period, about 540 million years ago. The trilobite pictured above
is an olenellid (probably in the genus Nevadella)
from the Early Cambrian of southwestern Nevada, a
very typical trilobite in the Lower Cambrian of North America.
It seems likely that trilobites were
preceded by soft-bodied ancestors: at several localities, sedimentary
rocks with trace fossils of trilobite activity underlie the oldest
rocks with trilobite body fossils.

Trilobites underwent several
radiations in the Cambrian; by the end of the Cambrian, about 500
million years ago, they were as diverse as they would ever be. In the
Ordovician, trilobite lineages began to specialize, and diversity
went down; it continued to drop in the Silurian and at the end of the
Devonian. The trilobite pictured below, Phacops, is
a typical Devonian trilobite, found in North America, Morocco, and
elsewhere.

In the Carboniferous and Permian Periods, trilobites became quite
scarce, but they did not go entirely extinct until the end of the
Permian, 245 million years ago, for reasons that are still not
understood.